A well known by-product of the rectification of an AC source of electrical power into a DC supply are the harmonics generated in the transformation and propagated through the electrical network back to the AC source. These harmonics are unwanted because they will increase circulating current for a given voltage and power therefore increasing the dissipation of power and cause distortion to the voltage which feeds other loads. The extra harmonics will create more circulating currents which are not consumed by the load. The AC source has to generate this power even though it is not used to drive the load. The additional current also means that the cabling has to be uprated to cope with the increased current. The parameter of power factor is widely used to measure such effects and is defined as the active power consumed divided by the active power plus the apparent power being generated. Ideally power factor needs to be unity. Increasing the power factor reduces the power factor. The typical conventional multi-phase rectifier circuit comprises a phase shifting transformer having multiple secondary outputs corresponding to the phase shift angles relative to the input. The output of the phase shifting transformer is supplied to banks of power cells, including a rectifier for each of the phase shift angles. The power cells each further comprise an inverter by which the multi-phase AC output is produced for use by the electrical load. The rectified power in the cells is supplied to each of the three phases of the load, for example an AC induction motor. For a given power requirement the phase shifting transformer is also known to be about larger than its simplest form of non-phase shifting transformer counterpart. However, the conventional transformer used to feed a rectifier comprising diode bridges will not improve the distorted current generated and they will need to be designed to be bigger in size and power rating to compensate for poor power factor.
Various module types have been proposed for the rectifier/inverter of multiphase power cells. A simple rectifier module has insulated gate bipolar transistors (IGBTs) for each leg connected across the phases and an accompanying DC link capacitor. The IGBTs take the full load rectified output of the rectifier circuit. This simple circuit has the benefit of low cost per power cell but has a relatively high input current harmonic content. An alternative approach is to replace the rectifying diodes in the above circuit with further active IGBT pairs per phase leg before the DC link capacitor. This solution has the benefit of being bi-directional and is able to limit the harmonic content by appropriate switching. However, the presence of further active devices in the form of the IGBTs has significant cost implications for this implementation.